Hot rolling of metal is a metal forming process that takes place at temperatures above the recrystallization temperature of the metal subjected to forming. This means that the rolling is performed at elevated temperatures, typically at temperatures above 700° C. Such high temperature during the rolling operation causes mechanical challenges for the equipment used in hot-rolling. The high temperature causes problems with hardness reduction of the roll material, therefore, the hot hardness of the roll is of utter importance in order to enable longer lifetime of the rolls.
In addition to the high temperature, the rolling sequence often comprises cooling of the rolled metal by subjecting the rolls to water, thereby causing large amounts of steam to be formed. The steam in combination with elevated temperatures causes severe oxidation of the rolling equipment used and especially the work rolls of the rolling equipment. The material used for the rolling rolls therefore needs to withstand high temperature without losing its hardness as well as a good abrasion/wear resistance at said temperatures and atmosphere.
Traditionally, the work rolls for hot rolling have been manufactured from high chromium nickel cast alloys. In most cases today work rolls for hot-rolling are composite rolls. The composite roll comprises a core with suitable mechanical properties, such as ductile iron or steel, and a sleeve with sufficient hot-hardness and sufficient wear resistance for the hot rolling.
The development of the outer layer of the roll have been very rapid since the beginning of the 1980's culminating in the applications of cast alloys containing Fe—C—Cr—W—Mo—V which replaced high chromium cast iron and Ni-hard cast iron. Alloys of this composition are generically called high speed steel.
The classical high speed steel exhibits both good hot-hardness and good wear resistance. In order to further improve the desired properties for hot rolling applications, the alloy design of the high speed steel is based on the composition of a so called M2 steel, wherein the main changes being higher carbon and vanadium content. A typical composition of such high speed steel often falls into the following ranges: 1.5-2.5% C, 0-6% W, 0-6% Mo, 3-8% Cr, and 4-10% V.
Basically, the essential target of a rolling mill plant is to keep the shape profile and surface roughness of the rolled metal as close as possible to the target values. The better performance of the high speed steel rolls in comparison to the previously used hot roll materials is related to the microstructural characteristics of the high speed steel such as a high amount of very hard and fine MC eutectic carbides and a base matrix hardened by secondary precipitated carbides.
Roll wear in hot-rolling is a complex process characterized by the concurrent operation of several surface degradation phenomena that involves at least: abrasion, oxidation, adhesion, and thermal fatigue. Thermal fatigue stems from stress developed by cyclic heating and cooling of a very thin boundary layer close to the roll surface. Adhesion comes from micro-welding regions of working metal into roll metal in the sticking zone of the roll gap. In the art, it is known that an increase of the volume fraction of eutectic carbides has a beneficial impact on the adhesive behaviour.
Oxidation of the roll during hot rolling markedly influences the wear behaviour of the roll material, since as long as this layer is smooth, adherent and continuous, it acts as a solid lubricant and as a thermal barrier, thus protecting the roll surface from degradation.
In U.S. Pat. No. 6,095,957, a roll for hot rolling with an outer layer comprising Fe—C—Mo—Nb—V is disclosed. This solution suggests that further improvement of the outer layer is possible.
In U.S. Pat. No. 4,941,251, a roll for hot rolling with an outer layer of ceramic is disclosed. However, this ceramic layer is brittle and hard to machine to the desired final dimensions of the working roll.